Ether alcohols, such as 2-butoxyethanol, have important industrial functions in such products such as cleaning supplies and coating materials. In the past, the manufacture of these products has been based on a process relying on a reaction between an alcohol and ethylene oxide. This conventional process has proven to be somewhat inefficient, in that it produces various undesirable byproducts along with the ether alcohols.
Monoether glycols can also be manufactured in a reaction between aliphatic aldehydes and ethylene glycol, instead of ethylene oxide, under acidic conditions in order to produce cyclic acetals. The acetal of ethylene glycol and butyraldehyde, for example, is described by Hibbert and Timm (Hibbert, H.; Timm, J. A. J. Am. Chem. Soc. 1924, 46(5), 1283-1290) and is achieved with a maximum yield of 50%. These cyclic acetals, or ketals when a ketone is substituted for the aldehyde, can then be subjected to hydrogenolysis in the presence of palladium and phosphoric acid catalysts. Such a process is described in U.S. Pat. No. 4,484,009.
The reaction of the polyhydroxyl compounds with aldehydes or ketones is an equilibrium reaction with the acetal product and by-product water. Yield of acetal or ketal is reduced via hydrolysis of the acetal by the co-product water. Thus, it is desirable to remove water from the reaction system to increase yield of the acetal.
The separation of water from the reaction mixture has been difficult since it often forms an azeotrope with the aldehyde reactants and with the cyclic acetal products. Entrainers have been employed to remove water through azeotropic distillation. Sulzbacher and coworkers, for example, describe removing the water by using benzene during the preparation of a number of acetals of ethylene glycol (Sulzbacher, M. et. al. J. Am. Chem. Soc. 1948, 70(8), 2827-2828). The environmental and health impact of benzene is an obvious concern in this method. Dessicants such as calcium chloride (DE 419223; Brönsted and Grove J. Am. Chem. Soc. 1930, 52(4), 1394-1403.) may be employed in the reaction vessel to remove water as it is formed, but disposal of the generated solid waste is an economic and environmental concern.
Another method as described by Astle and coworkers, involves heating the glycol and aldehyde over an heterogeneous acidic resin and distilling out the acetal and water as they are formed (Astle, M. J. et al, Ind. Eng. Chem. 1954, 46(4), 787-791). This method generally had low yields with one example for the manufacture of 2-butoxyethanol reported as having a yield of about 92% using a molar ratio of ethylene glycol to butyraldehyde of about 1.3:1. In these reactions, water was being separated from the reaction mixture in the flask as the water was being formed, and upon completion, the reaction mixture in the flask was filtered and phase separated. The removal of water from the reaction mixture as it was being formed follows from the understanding that the reaction of the polyhydroxyl compounds with aldehydes is an equilibrium reaction with the acetal product and by-product water, and the yield of acetal is reduced via hydrolysis of the acetal by the co-product water or can be increased with the removal of water as it is formed.
One pot reaction systems have also been reported, that is, reacting an aldehyde and a polyhydroxyl with hydrogen in the presence of a noble metal catalyst directly to the desired ether alcohol. For example, U.S. Pat. No. 5,446,210 describes a process for the production of hydroxy ether hydrocarbons in a one pot system by reacting a polyhydroxyl with an aldehyde and hydrogen in the presence of a noble metal catalyst where the molar ratio of polyhydroxyl to aldehyde compound ranges from 5:1 to 1:5 is described, but with these molar ratios, the yield was low in the range of 35 to 50% when including the bis-types of by-products with low selectivity to the mono-ether products.
US Publication No. 2010/0048940 also describes a one pot system in which a polyhydroxyl and a aldehyde compound and hydrogen are reacted together in the presence of a hydrogenolysis catalyst to provide the polyhydroxyl ether, where the molar ratio of polyhydroxyl to aldehyde exceeds 5:1 to improve selectivity and yield. An example of a two stage process in which the acetal compound was first synthesized and then subjected to hydrogenolysis was reported without describing the yield value of the acetal produced, although the yield to the 2-butoxyethanol by hydrogenolysis of the acetal was reported as having a selectivity of about 61%.
In U.S. Pat. No. 5,917,059 to BASF Aktiengesellschaft, the authors generate cyclic acetals and ketals by reacting a molar excess of aldehydes and ketones with polyhydroxyl compounds in the presence of an acid catalyst. The water is removed by continuously distilling unreacted aldehydes or ketone starting materials, thus co-distilling the formed water in the water/aldehyde azeotrope, and further replacing the distilled aldehyde or ketone with fresh aldehyde or ketone. The aldehydes and ketones act not only as a reactant but also as a medium for transporting the water produced in the reaction. This method requires large excess of aldehyde (e.g. 4:1 molar ratio of aldehyde:alcohol) to be successful.
Reactive distillation is employed in U.S. Pat. No. 6,015,875 and U.S. Pat. No. 7,534,922 B2 to generate low boiling acetals. The authors make use of heterogeneous acids in the packing of the column and feed low boiling starting materials such as methanol, ethanol, formaldehyde, and acetaldehyde. The formed acetals are removed overhead above the distillation reaction zone and the co-product water is removed below the distillation reaction zone. This method limits the types of usable reactants to those producing materials that boil at a temperature lower than water.
As can be seen from the available literature, there exists a continued need to produce cyclic acetal or ketal compounds in high yield using a simple economic process.